Bulk polymerized aromatic hydrocarbon in rubber and polyphenylene ether

ABSTRACT

A resin improved in impact resistance is obtained by continuously subjecting to bulk-polymerization a mixture composed mainly of a vinyl aromatic compound and containing a rubbery substance; incorporating a mixture comprising an aromatic hydrocarbon and a polyphenylene ether into the system during a period from the stage immediately after the phase inversion of the rubbery substance to the stage where the total polymer concentration has become 40 percent, and further continuing the bulk-polymerization with thorough stirring until the polymerization is substantially completed. The thus obtained resin has improved physical properties and gives a molded article excellent in appearance.

United States Patent N akanishi et al.

[ 1 May 23, 1972 [54] BULK POLYNIERIZED AROMATIC HYDROCARBON IN RUBBERAND POLYPHENYLENE ETHER [72] Inventors: Atsuo Nakanishi, Kanagawa-ken;Shinichi Izawa, Tokyo; Mikio Sato, Yokohama, all of Japan [73] Assignee:Asahi Dow Limited, Tokyo, Japan [22] Filed: Nov. 17, 1969 [21] Appl.No.: 877,499

[30] Foreign Application Priority Data Nov. 20, 1968 Japan ..43/84449[52] US. Cl. ..260/4 AR, 260/29.6 NR, 260/876 R,

260/878 R, 260/880 R [51 Int. Cl. ..C08c 9/14, C08d 9/08 [58] Field ofSearch .260/4 AR, 892, 4, 880, 876

[56] References Cited UNITED STATES PATENTS 3,487,127 12/1969 Erchak eta] ..260/876 Primary Examiner-Murray Tillman Assistant ExaminerJ.Seibert Attorney-Cushman, Darby & Cushman ABSTRACT The thus obtainedresin has improved physical properties and gives a molded articleexcellent in appearance.

5 Claims, No Drawings BULK POLYMERIZED AROMATIC HYDROCARBON IN RUBBERAND POLYPI-IENYLENE ETI'IER This invention relates to an improvedprocess for preparing modified plastics containing rubbers. Moreparticularly, the invention pertains to an improved method for producingimpact resistant resins, characterized in that a starting mixturecontaining a rubbery substance and a vinyl aromatic compound iscontinuously subjected to bulk-polymerization; a mixture comprising anaromatic hydrocarbon and a polyphenylene ether represented by theformula:

R1 l l L I l R2 n wherein R and R represent individually an alkyl grouphaving one to four carbon atoms or a halogen atom, and n represents thedegree of polymerization, is added to the system during a period fromthe stage immediately after the phase inversion of the rubbery substanceto the stage where the polymer concentration has become 40 percent; andthe bulk-polymerization is further continued until the polymerization issubstantially completed, thereby improving the physical properties ofthe polymerization product and the appearance ofa molded articleobtained therefrom.

It is well known that a mixed resin obtained by blending a polystyrenewith a rubbery substance is very different in physical properties from amixed resin obtained by polymerizing a styrene monomer in the presenceof a rubbery substance, due to differences of the two in the bondedstate and compatibility between the polystyrene portion and the rubberysubstance and in the dispersed state of the rubber component. It isfurther known that when an improvement in impact strength is desired,the latter, i.e. the resin prepared according to socalledgraft-polymerization process, is advantageously. However, it is alsowell known through experiments that even when said graft polymerizationprocess is adopted, the shape, size and particle size distribution ofthe dispersed rubber component and the micro structure of rubberparticles vary due to more or less differences in polymerizationconditions and affeet the physical properties, such as tensile strength,flexural strength, impact resistance and elongation of the resultingmixed resin; and the appearance, such as surface gloss, etc., of amolded article obtained therefrom. For the attainment of desiredphysical properties, by controlling the shape, size and particle sizedistribution of the dispersed rubber component, therefore, anindustrially effective means is to vary, for example, inbulk-polymerization, the polymerization rate, the stirring speed, or theshearing force at stirring.

When an ordinary commercial production plant is used, however, no greatvariation in the dispersed state of the rubber phase can be expected,because there is a certain limit in mechanical or operational ability ofthe plant. If the aforesaid procedures are forcibly effected, not onlythe dispersed state of the rubber but also other factors affecting thephysical properties of the resulting resin, e.g. the normal state of thepolystyrene used as matrix, become unbalanced to make it impossible toattain the desired object, i.e. improvement in physical properties andappearance. Several examinations have heretofore been made as in regardthe phenomenon that a rubber, which is present as solute in styrene atthe beginning of the polymerization, is gradually changed in phase withthe progress of the polymerization and, at a certain stage, iscompletely converted to a dispersed phase in the form of particles.However, the course from such phase inversion to the finalpolymerization product is complex, and it has been quite difficult toproperly vary the two phases of the dispersed particulate rubber and thedispersion medium polystyrene. These are fundamentally important factorsfor said physical properties and appearance.

In view of such circumstances as mentioned above, the present inventorsmade a wide scope of comparisons and examinations in effectiveness ofbulk-polymerization processes,

in which operational conditions, such as stirring speed, temperature,conversion and the like, were varied, or other components such asvarious solvents or polymers were added to the polymerization systems.As the result, we have found that a process, in which a polyphenyleneether, a type of noncrystalline polymer having high transitiontemperatures (Tg), is added during the polymerization, imparts markedlygreat effects. The present invention results from the above-mentionedfinding. That is, when, in the preparation of a rubber-modified impactresistant polystyrene, a polyphenylene ether is additionallyincorporated into the system during a period from the stage immediatelyafter the phase inversion of a rubbery substance to the stage where thepolymer concentration has become 40 percent, preferably 15-35 percent,and the incorporation time, amount, concentration and molecular weightof the polyphenylene ether are optionally varied, the resultingpolystyrene can be processed, without being changed in tensile strength,impact resistance, flow property, elongation, etc., into a moldedarticle which has been far more improved in gloss, smoothness and thelike appearance than is the case with conventional process. What isimportant is that the present process gives a resin which, unlike aresin according to the conventional process, does not suffer from suchunbalance in physical properties that the resin is greatly lowered inimpact strength when processed into a molded article improved in glossor is deteriorated in heat resistance and tensile strength whenprocessed into a molded article improved in smoothness or strength ofwelded portion. Such a process and its effects can never be derived fromconventional knowledge and experience of those skilled in the art. Theprocess of the present invention is an epoch-making process forpreparing impact resistant resins.

For example, there was carried out an experiment in which there wasprepared a polymer according to the present process by continuouslyfeeding a styrene solution containing 5 percent of polybutadiene to abulk-polymerization vessel, a styrene solution ofpoly(2,6-dimethylphenylene-l,4-ether) was added to the system at thestage where the conversion had reached 35 percent, so that the finalconcentration of the polybutadiene became 4 percent and that of the poly(2,6- dimethylphenylene-l,4-ether) became 15 percent, and then thepolymerization was completed. In this case, the impact resistance of thepolymer in terms of Izod impact value was about 19 kg.cm/cm and wasabout 3 times the value of a polymer containing nopoly(2,6-dimethylphenylene-1,4- ether). The polymer containing saidpolyphenylene ether showed great differences from the one containing nopolyphenylene ether in surface gloss molded articles when these weremolded under the same conditions. This is considered ascribable to thefact that the rubber particles dispersed in the polymer containingpolyphenylene ether became smaller. In the above-mentioned case, thediameter of rubber particles dispersed in the resin was about 0.1;.t0.3;; and was about 1/10 the diameter of rubber particles in a highimpact polystyrene resin. According to the conventional process it hasbeen extremely difficult or impossible to attain such a high impactstrength with such a small rubber particle diameter. Further, when thesame polyphenylene ether is added to a polystyrene containing no rubber,slight improvement in impact strength is observed. Thus, it is clearthat in the present process, the polyphenylene ether affects the rubberin the polymer in such a manner as to display a synergistic effect. Thisis supported by the experiments mentioned below. The aforesaid fact thatthe dispersed rubber particles have been made extremely fine may beregarded as an example of such synergistic effect. According to thepresent process there was prepared an impact resistant polystyrene mixedresin containing 20 percent of a polyphenylene ether and 5 percent ofrubber. The resin portion of said mixed resin was extracted with tolueneand the proportions of polystyrene and polyphenylene ether in theextract were investigated to find that the proportion of the former was88 percent and that of the latter was 12 percent. For comparison apolyphenylene ether and a rubber-containing polystyrene were blendedtogether to form a mixture having the same composition as mentionedabove and the resin portion of the mixture was extracted with toluene.The proportions of polystyrene and polyphenylene ether in the extractwere 65 percent and 35 percent, respectively. From this it was clearthat according to the present process a considerable amount ofpolyphenylene ether had been incorporated into the rubber particleportion and had been brought into a non-extractible state. Such a statecannot be attained in the case of a mere blend of a rubber-containin gimpact resistant polystyrene with a polyphenylene ether.

In a continuous bulk-polymerization process it is a conventionalprocedure to continuously feed a styrene monomer solution containing arubber component to a first polymerizer to form a polystyrene at atemperature of 70 170 C. At the same time the rubber component, whichwas initially been present as a continuous phase, is converted to adispersed phase by employing a suitable stirring means. in this case thestirring facilitated the removal of polymerization heat generated in thesystem to make uniform the temperature distribution in the directionperpendicular to the polymer flow. The stirring also facilitated thedispersing of the rubber particles by means of shearing force, asmentioned above. After the conversion has reached 25-40 percent, thepolymer mixture is sent to a second continuous polymerizer having adifferent temperature distribution and stirring speed to complete thepolymerization.

In practicing the process of the present invention a polyphenylene etheror a mixture containing said ether is fed to the first polymerizerduring a period from the stage immediately after the phase inversion ofthe rubber component to the stage where the polymer concentrationbecomes 40 percent, i.e. optionally at the latter-half portion of thefirst polymerizer or at the stage immediately after completion ofpolymerization in the first polymerizer. lf, in this case, the additiontime, kind and molecular weight of the polyphenylene ether and thetemperature of reaction mixture are suitably selected, it is possible tooptionally vary the balance between the physical properties and flowproperty of the resulting polymer and the gloss and the like appearanceof a molded article obtained therefrom. in case the polyphenylene etheris present in the polymerization system from the beginning ofpolymerization, the behavior of rubber in inversion from the continuousphase to the dispersed phase is entirely different from that observed inthe present process and, in view of the shape and internal microstructure of the rubber particles and the physical properties andappearance of the resulting polymer, the above-mentioned case isobviously distinguished from the process of the present invention.

It is impossible to say in regard to the surface gloss, smoothness andthe like appearance of a molded article obtained from rubber-modifiedpolystyrene with molding conditions, that any method of quantitativeexpression has been established, so that a judgement by visualobservation is the most accurate. As a surface glossmeasuring equipment,there has been used a glossmeter, which quantitatively expresses thesurface gloss in terms of the reflectivity which is the per cent oflight of incidence that has been reflected. Although the glossmeter isnot high in accuracy, it has been confirmed that a value represented bysaid reflectivity substantially corresponds to evaluation by visualobservation. In each of the examples set forth hereinafter, therefore,the light reflectivity was used to express the surface state.

It is well known that in the injection molding of a thermoplastic resin,in general, the appearance of the resulting molded article becomesbetter when they are adopted such conditions that the molding cycle timeis increased to sufficiently stabilize the molded article. Foreconomical reasons, however, a shorter cycle time is required inmolding. Accordingly, the advent of a resin, which, even when subjectedto severe molding conditions, can always give a molded article excellentin appearance, has been desired.

In order to show the superiority of the resin obtained according to thepresent invention by the additional incorporation of a polyphenyleneether, the molding cycle time employed in molding the resin of thepresent invention was compared with that employed in molding arubber-containing impact resistant polystyrene obtained under the samepolymerization conditions as in the case of the present resin, exceptthat no polyphenylene ether was additionally incorporated. Thecomparison was effected in such a manner that the two resins wereindividually molded so that the resulting molded articles were identicalin surface state when evaluated in terms of visual observation and lightreflectivity, and the mold temperature was lowered to shorten thecooling time. As a result, the molding cycle time required for thepresent resin was 4/5 to /5 the time required for the rubber-modifiedpolystyrene. This value obviously is of extremely great significancefrom the industrial standpoint.

For the preparation of rubber-modified polystyrenes, abulk-polymerization process and a bulk-suspensionpolymerization processare known, but the former is frequently adopted for economical reasons.

Dutch Pat. Application No. 66-17529, Rexall Drug & Chem. Co., disclosesa process for preparing a mixed resin by subjecting styrene topolymerization in the presence of rubber, adding a polyphenylene etherto the polymerization system during said polymerization, immediatelythereafter converting the system to a suspension system and theneffecting suspension polymerization to complete it. According to abulksuspension-polymerization process there is slight agitation in thesuspended particles formed in the latter-half stage of thepolymerization, i.e., in the suspension-polymerization stage, and it isextremely difficult to finely divide the rubber particles dispersed inthe suspended particles.

in the present invention, only bulk-polymerization is effected from thebeginning to the end of the process, and hence the effect obtained isthat a strong shearing force acts continuously on the dispersed rubberparticles to make the rubber particle diameter extremely small (about0.1 0.3 ,u), so that the resulting resin can give a molded articleexcellent in appearance.

As the rubbery substances to be added to the polymerization system inthe present invention, there may be used natural rubber, syntheticrubbers derived from conjugated dienes, and other synthetic substancesshowing rubber-like behaviors. These include, for example, natural creperubber, butadienestyrene copolymer rubber, butadiene-acrylonitrilecopolymer rubber, polybutadiene rubber, polyisoprene rubber andethylene-propylene rubber.

Examples of the vinyl aromatic compound referred to in the presentinvention include styrene, vinyltoluene, vinylxylene, ethylvinylbenzene,isopropenylbenzene, isopropylstyrene and ethylvinyltoluene. In additionto these, there may be contained as copolymen'zation components othercopolymerizable vinyl compounds.

Examples of the polyphenylene ethers employed in the present invention,which are incorporated into the polymerization system and show markedaction for the improvement in properties of the resulting resin, includepoly(2,6-dimethylphenylene-1 ,4-ether), poly( 2,6- diethylphenylene-l,4-ether), poly(2,6-dichlorophenylene- 1,4-ether), poly(2,6dibromophenylene-l,4-ether), poly(2- methyl-o-ethylphenylene-l,4-ether), poly(2-chloro-6- methylphenylene- 1,4-ether), poly(2-methyl-6-isopropylphenylenel ,4-ether),poly(2,o-di-n-propylphenylene-l ,4-ether), poly(2-bromo-6-methylphenylenel ,4-ether) poly( 2-chloro- 6-bromophenylene-l,4-ether) and poly( 2-chloro6- ethylphenylene-l ,4-ether).

in the present invention, the mixture containing aromatic hydrocarboncompound and polyphenylene ether which is added after the firstpolymerization may be incorporated with an organic peroxide or anorganic hydroperoxide. In this case, there are attained such advantagesthat there is promoted the mutual reaction of rubbery substance with theresidual vinyl aromatic compound and the polymerization rate isincreased, and greater influence can be brought about on the physicalproperties of the resulting resin.

in practicing the present process, a pre-polymerizer may be attached tothe first polymerizer; the first and second polymerizers may further besubdivided into several reaction zones; or a chain transfer agent andother suitable modifiers and a solvent may be added at an optional stageduring the polymerization. All these procedures are also involved in thetechnical scope of the present invention.

The impact resistant polymer compositions obtained according to theprocess of the present invention have such advantage over theconventional rubber-modified styrene type impact resistant resins thatmolded articles obtained therefrom can be greatly improved in surfacegloss, smoothness and the like appearance without sacrificing theprocessability, tensile strength, impact resistance and heat resistanceof the compositions. The process of the present invention gives resincompositions capable of producing an extremely wide scope of usefulproducts.

The present invention is illustrated in further detail below withreference to examples, in which all the parts are by weight.

EXAMPLE 1 A homogeneous mixture comprising 80 parts of styrene monomer,8 parts of styrene-butadiene rubber, 2 parts of mineral oil and 10 partsof ethylbenzene was continuously fed to a first polymerizer understirring at 30 r.p.m. to effect bulkpolymerization. The polymerizationproceeded while controlling the temperature and the feed rate so thatthe total solid content of the mixture became 25 percent at the exit ofthe first polymerizer. The mixture from the first polymerizer was mixedwith a homogeneous mixture comprising parts of styrene monomer, 10 partsof ethylbenzene and 10 parts of poly(2,6-dimethylphenylene-1,4-ether)and was introduced into a second polymerizer to effect polymerization,and the polymerization was substantially completed at the exit of thesecond polymerizer. The feed rate of the additionally incorporatedmixture was so controlled as to become 4/10 the feed rate of the mixtureintroduced into the first polymerizer. The resulting resin had a rubbercontent of 6.5 percent, a polyphenylene ether content of 8.1 percent, atensile strength of 440 kg/cm and an lzod impact strength of 15.6kg.cm/cm. Further, the heat distortion temperature of the resin was 1 14C., which is about 10 C. higher than in the case of an impact resistantresin containing no polyphenylene ether. According to microscopicobservation, the dispersed rubber particle diameter in the above casewas 0.2 0.3 p.. The resin obtained in the example and arubber-containing polystyrene obtained under the same conditions as inthis example, except that no poly(2,6-dimethylphenylene-1,4-ether) wasadditionally incorporated, where individually subjected to injectionmolding at a screw temperature of 200 230 C. and a mold temperature of40 50 C. so that the surface gloss of each molded article became 80percent in terms of light reflectivity, and the times of individualmolding cycles were measured. As the result, the time of the moldingcycle of the resin incorporated with the polyphenylene ether was in therange of 20-30 seconds and that of the resin containing no polyphenyleneether was in the range of40-50 seconds.

EXAMPLE 2 A homogeneous mixture comprising 78 parts of styrene monomer,10 parts of polybutadiene, 2 parts of mineral oil and 10 parts ofethylbenzene was continuously fed to a first polymerizer under stirringat 30 r.p.m. to effect bulkpolymerization. The polymerization proceededwhile controlling the temperature and the feed rate so that the totalsolid content of the mixture became percent at the exit of the firstpolymerizer. The mixture from the first polymerizer was thoroughly mixedwith a homogeneous mixture comprising 30 parts of ethylbenzene and 30parts of poly(2,6- dimethylphenylene-1,4-ether) and was introduced intoa second polymerizer to effect polymerization. The polymerization wassubstantially completed in the second polymerizer, and the resultingresin was taken out. The feed rate of the mixture additionallyincorporated at the intermediary stage was so controlled as to become6/10 the feed rate of the mixture introduced into the first polymerizer.The thus obtained resin has a tensile strength of 540 kglcm an lzodimpact strength of 25.5 kg.cm/cm and a heat distortion temperature of130 C. The resin obtained in this example and a rubber-containingpolystyrene obtained under the same conditions as in this example,except that no poly(2,6-dimethylphenylene-1,4-ether) was additionallyincorporated, were individually subjected to injection molding at ascrew temperature of 220 240 C. and a mold temperature of 70 75 C. sothat the surface gloss of each molded article became percent in terms oflight reflectivity, and the times of individual molding cycles weremeasured. As the result, the time of the molding cycle of the resinincorporated with the polyphenylene ether was in the range of 18-20seconds and that of the resin incorporated with no polyphenylene etherwas in the range of 30-35 seconds.

EXAMPLE 3 A homogeneous mixture comprising 83 parts of styrene monomer,5 parts of styrene-butadiene copolymer rubber, 2 parts of mineral oiland 10 parts of ethylbenzene was continuously fed to a first polymerizerunder stirring at 30 r.p.m. to effect bulk-polymerization. At the middlestage of polymerization in the first polymerizer where the total solidconcentration of the mixture had become 15 percent, a mixture comprising25 parts of styrene monomer, 10 parts of ethylbenzene and 5 parts ofpoly(2,6-dimethylphenylene-l,4-ether) was additionally incorporated intothe first polymerizer, and the temperature and the feed rate werecontrolled so that the total solid content became 30 percent at the exitof the first polymerizer. Subsequently, the mixture was directly fed toa second polymerizer under stirring at a speed of 7 r.p.m. to completethe polymerization, and the resulting resin was taken out. The feed rateof the additionally incorporated mixture containing the polyphenyleneether was so controlled as to become 6/10 the feed rate of the startingmixture introduced into the first polymerizer. The thus obtained resinhad a rubber content of 4.1 percent, a polyphenylene ether content of4.0 percent, a tensile strength of 420 kg/cm and an Izod impact strengthof l 1.2 kg.cm/cm. According to microscopic observation, the averageparticle diameter of the dispersed rubber in the above case was 0.2 Theresin obtained in this example and a rubber-containing polystyreneobtained under the same conditions as in this example, except that nopo1y( 2,6-dimethylphenylene-1,4-ether) was additionally incorporated,were individually subjected to injection molding at a screw temperatureof 190 220 C. and a mold temperature of 50 60 C. so that the surfacegloss of each molded article became 85 percent in terms of lightreflectivity, and the times of individual molding cycles were measured.As the result, the time of the molding cycle of the resin incorporatedwith the polyphenylene ether was in the range of 30-35 seconds and thatof the resin incorporated with no polyphenylene ether was in the rangeof 4050 seconds.

EXAMPLE 4 Example 1 was repeated, except that a mixture of 85 parts ofstyrene monomer, 3 parts of polybutadiene, 2 parts of mineral oil and 10parts of ethylbenzene was used as the starting mixture to be chargedinto the first polymerizer. The resulting resin has a rubber content of2.2 percent and a polyphenylene ether content of 7.9 percent, and had atensile strength of 390 kg./cm and an Izod impact strength of 9.5kg.cm/cm. The resin obtained in this example and a rubbercontainingpolystyrene obtained under the same conditions as in this example,except that no poly(2,6-dimethylphenylenel,4-ether) was additionallyincorporated, were individually subjected to injection molding at ascrew temperature of 200 230 C. and a mold temperature of 60 70 C. sothat the surface gloss of each molded article became 90 percent in termsof light reflectivity, and the times of individual molding cycles weremeasured. As the result, the time of the molding cycle of the resinincorporated with the polyphenylene ether was in the range of 20-25seconds and that of the resin incorporated with no polyphenylene etherwas in the range of 35-40 seconds.

EXAMPLE Example 3 was repeated, except that the additionallyincorporated mixture containing the polyphenylene ether had been chargedwith 0.05 part of benzoyl peroxide. The resulting resin was identical incomposition with that obtained in Example 3, but came to have a tensilestrength of 485 kg./cm and an Izod impact strength of 8.5 kg.cm/cm. Theresin obtained in this example and a rubber-containing polystyreneobtained under entirely the same conditions as in this example, exceptthat no poly( 2,6-dimethylphenylene-l,4-ether) was additionallyincorporated, were individually subjected to injection molding at ascrew temperature of 190 220 C. and a mold temperature of 50 60 C. sothat the surface gloss of each molded article became 85 percent in termsof light reflectivity, and the time of individual molding cycles wasmeasured. As the result, the time of the molding cycle of the resinincorporated with the polyphenylene ether was in the range of 25-30seconds and that of the resin incorporated with no polyphenylene etherwas in the range of 40-45 seconds.

What is claimed is:

1. In a process for producing an impact resistant resin, which comprisesdissolving a rubbery material in a mixture composed mainly of a vinylaromatic compound and subjecting the resulting solution to thermalpolymerization in the presence of a polyphenylene ether, the improvementwhich comprises heating a mixture solution containing 3.0 to l0 parts byweight of a rubbery material selected from the group consisting ofnatural crepe rubber, butadiene-styrene copolymer rubber,butadiene-acrylonitrile copolymer rubber, polyisoprene rubber,polybutadiene rubber and ethylenepropylene rubber and 78 to parts byweight of a vinyl aromatic compound at a temperature of 70-l70 C. tobulkpolymerize the mixture, adding to the polymerization mixture apolyphenylene ether represented by the formula:

'- z .JII

wherein R, and R are alkyl of one to four carbon atoms or halogen and nis the degree of polymerization, in a proportion of 4.0 to 20 percent byweight based on the total weight of the final polymer at a time of fromthe stage immediately after the formation of a dispersoid of the rubberymaterial to the stage at which the total polymer concentration reaches40 percent by weight, and continuing the bulk-polymerization of theresulting mixture with a sufficient stirring at a temperature of 70 l 70C. to complete the polymerization.

2. A process according to claim 1, wherein the vinyl aromatic compoundis selected from the group consisting of styrene, vinyltoluene,vinylxylene, ethylvinylbenzene, isopropenylbenzene, isopropylstyrene andethylvinyltoluene.

3. A process according to claim 1, wherein the polyphenylene ether ispoly(2,6-dimethylphenylene-l,4-ether).

4. A process according to claim 1, wherein the aromatic hydrocarbon isstyrene or ethylbenzene.

5. A process according to claim 1, wherein the solution of thepolyphenylene ether is incorporated into the system at the stage wherethe total polymer concentration has become 15-35 percent,

2. A process according to claim 1, wherein the vinyl aromatic compoundis selected from the group consisting of styrene, vinyltoluene,vinylxylene, ethylvinylbenzene, isopropenylbenzene, isopropylstyrene andethylvinyltoluene.
 3. A process according to claim 1, wherein thepolyphenylene ether is poly(2,6-dimethylphenylene-1,4-ether).
 4. Aprocess according to claim 1, wherein the aromatic hydrocarbon isstyrene or ethylbenzene.
 5. A process according to claim 1, wherein thesolution of the polyphenylene ether is incorporated into the system atthe stage where the total polymer concentration has become 15-35percent.